This invention relates to the field of chemistry. More particularly, this invention relates to the field of protective water repellent coatings. This invention includes methods for coating optical elements to provide protective, water repellent surfaces, and the resulting coated articles. By optical element is meant an element such as a lens or windshield made of light or electromagnetic radiation transmitting material such as glass.
It is known in the optical arts to coat lenses, etc. with inorganic substances in order to enhance performance of the lens. Magnesium fluoride (MgF.sub.2), for example, has been employed as a coating to impart antireflective qualities to a lens and thus increase its transmissivity, or light gathering capabilities.
Optical glass surfaces exposed to rain or sea water cannot transmit images without distortion if water accumulates on the surface. Further, if water is allowed to freeze, further distortion is produced. Also, accumulated ice may reduce flight performance if the optical element makes up the forward portion of a flight vehicle. Coating compositions are known which render a glass optical element water repellent. Many of these repellent coatings, however, are not effective on optical elements which are coated with magnesium fluoride or other material. Optical elements having such coatings are not only subject to the same degradation of performance as the uncoated elements from water exposure but the salt remaining from evaporated salt water attacks the coating materials.
It is known in the art to provide water repellent compositions for coating optical elements having magnesium fluoride coatings. One such composition comprises a mixture of hydrocarbon wax and mixed isomers of long chain alkyl or alkenylsuccinic acid long chain alkylmonoamides. It is desirable, however, to provide a coating formulation having improved durability.
Another requirement for water repellent coating compositions has arisen due to the development of optical systems having electrically conductive indium-tin oxide (ITO) coatings. ITO coatings are composed of a mixture of indium oxides and tin oxides which have variable electrical resistance depending upon the oxidation state of the materials. Such a coating is useful for coating sensor optics in guided missile applicatons. No prior art repellent coatings are known to be effective or compatible with ITO coatings.